The present invention relates to an apparatus for testing heads on a Head Stack Assembly (xe2x80x9cHSAxe2x80x9d) or single head suspension. More particularly, the present invention relates to a multi-coil assembly for testing heads.
Each hard disk or hard file contains a magnetic recording head for reading from and writing to the hard disk. The head is a part of a head stack assembly (xe2x80x9cHSAxe2x80x9d) that contains the air bearing surface that allows the head to fly close to the hard disk. The head also contains the elements needed to write magnetic transitions on a hard disk and the elements needed to read magnetic transitions previously written by a head. A typical write element contains thin film conductors looped inside soft magnetic pole pieces. A typical read element is a magneto-resistive (MR) strip that changes resistivity as a magnetic field is applied.
During the manufacturing of magnetic disk drives or hard files, the heads must be tested to determine whether the heads are functioning properly. A properly functioning magnetic recording head must be able to read from a hard disk and write to a hard disk. For many years, external magnetic coils have been used to test the read elements of a head. These coils apply a magnetic field to the read element, thereby attempting to energize the read elements, which provides an indication of functionality. The early external magnetic coils were very large and one pair of coils would excite an entire stack of heads. These large coils could excite the head read elements, but they could not read the small magnetic field from one head when the head was writing. The large coils were further limited in capability by the low frequency response inherent in a large structure.
The next generation of head testers were smaller and could fit between head pairs in a stack of heads. Early versions of these head testers could only excite the head""s read element, later versions could also read as the head would write. FIG. 1 illustrates this next generation head tester. As shown in FIG. 1, a printed circuit board with a coil wrap having a plurality of turns is attached to the printed circuit board. There are two coil pads to facilitate connection to a testing device.
A number of limitations were uncovered with this type of head tester and similar head testers in the industry: (1) the head testers were inconvenient to use to test both the read and write elements of a head; (2) the head testers did not enable a testing device to accurately determine the sensitivity of the read/write elements of a head; (3) the head tester could not be used to enable a testing device to determine whether a head was incorrectly wired; (4) the head tester could not be used to enable a testing device to determine whether the spacing between the head and the coil on the head tester was within an acceptable range of distances; and (5) the head tester could not be used to determine whether the heads in a head stack assembly were properly aligned.
First, the head testers shown in FIGS. 1 and 2 are inconvenient to use to test both the read and write elements of a head. An important aspect to note in FIG. 1 is the horizontal location of the MR read elements with respect to the central axis of the coil. The MR elements are located on the axis and in this position the MR read elements are most responsive to the applied magnetic field. However, the location of the coil axis and the head, shown in FIG. 1, needs to be changed when the head is writing. If the coil in FIG. 1 is used to read the magnetic field from the head""s thin film write (xe2x80x9cTFWxe2x80x9d) elements, the magnitude of the magnetic field is symmetrical about the axis of the coil. This field produces electric potentials in the windings of the coil that cancel each other. This cancellation results is no voltage being present at the coil pads when the head writes while the head tester is positioned as shown in FIG. 1. FIG. 2 shows the horizontal location of the head""s TFW element (i.e., the write element) where the voltage read at the coil pads is optimum. In FIG. 2, the TFW element is located over (or under) one side of the coil to allow the electric potentials in the windings not to cancel each other. The distance between the TFW element (i.e., the write element) and the MR element (i.e., the read element) of one head is very small and for this discussion they are assumed to be at the same location on the head. The result is that if both reading by the head and reading by the coil are to be performed, either the heads or the coils must be relocated horizontally between the two modes of reading. Thus, either the heads or the coil needs to be horizontally relocated when they are being used in a test application that requires both read modes. The time needed to move the heads (or the head tester) between the two read modes increases the overall test time and negatively effects the positional accuracy, thereby inconveniencing the user.
Second, the head testers shown in FIGS. 1 and 2 can only make approximate measurements of the read and write elements sensitivity due to the uncertainty of the vertical spacing between the coil and the head""s read or write element. The spacing between the head and coil varies due to head tilt, separation of the head from the suspension load dimple, and dimensional tolerances between the head separator and the coil. Variations of 30 to 50% can be observed in the voltage""s being read as the spacing changes over the range allowed by current HSA and coil technology. Thus, a tester that can enable a more accurate determination of the sensitivity of the read and write elements of a head is desirable.
The third problem is the use of one coil on a head tester for two heads. If the head selection circuitry is improperly assembled, the one coil can not detect the case where the two head selection lines are reversed at the same time the polarity of the head element is reversed. This type of assembly error commonly occurs during production. Thus, a tester that can enable a determination of whether the head selection circuitry is properly assembled would be desirable.
The fourth problem exists when a head is mishandled at a previous manufacturing step resulting in a spacing between the head and the coil that falls outside some acceptable range. The head tester shown in FIGS. 1 and 2 can only sense that a combination of head sensitivity and/or head spacing is causing the extra high or low signals being read. Thus, a tester that can enable a determination of whether the spacing between a coil on a head tester and a head is in an acceptable range of distances is desirable.
The fifth problem involves the precise alignment of the center of each coil in a stack of coils. The coils are to be co-axially aligned over the entire length of the stack of heads. A precise alignment of the coils in a stack of testers enables gathering positional information relating to head alignment in the HSA. Without precise alignment, the coils can not be used to determine the mechanical location of the head read/write elements in the plane of the hard disk. The head tester shown in FIGS. 1 and 2 does not provide a xe2x80x9cdesigned inxe2x80x9d method of alignment. Thus, producing a stack of head testers in which the coils are precisely aligned to enable the gathering of positional information related to head positions in the HSA is desirable.
The present invention relates to a head tester for testing heads having a read element and a write element. The tester comprises a substrate on which a read coil is positioned to be sensitive to the write element of the head. A write coil is also formed on the substrate and this coil is positioned such that the read element of the head to be tested is sensitive to the write coil.
This head tester assists in the testing of head stacks (or single head suspension) assemblies. These tests performed by the tester provide an indication of the functionality of the heads.